(Eglin AFB FL)
My opinion only ………. but ………..
After reading your experience above, I would like to share my perspective for what it may be worth.
I have experience designing batteries for the defense industry (missiles and standoff weapons) and telecommunications industry (telephone central offices) and believe your issue is a mismatched application of your GC2 battery stack.
Deep cycle batteries, like all batteries, are designed and manufactured with a specific application in mind. Automotive batteries are crank batteries designed to put out hundreds of amps for a very short duration and intend to be fully recharged with the same high current going in as that that just came out. If you use a crank battery for a deep cycle application (eg: trolling motor) it will work temporarily but will not provide long term satisfaction since the crank battery plates are thinner and are damaged by long slow discharging and recharging. Deep cycle batteries are designed with thicker anode plates to compensate for the long slow discharging and recharging cycles.
Side note regarding battery operation …….. a battery is nothing more than an energy exchange device. A full battery has all of its available oxygen collected and stored on its anode (positive terminal) and transfers the oxygen molecules to the cathode (negative terminal) during discharge. Recharging a battery does little more that move the oxygen molecules from the cathode back to the anode. As the oxygen molecules move from the cathode back to the anode (negative plates to the positive plates through the dielectric insulator), small particles from the plates typically move with the oxygen which over time settles into the bottom of the battery eventually shorting out some of the plates, causing an apparent dead or degraded battery. A lot of the battery manufacturers are aware of this and offer gel cell and absorbed glass matt battery designs to compensate for this. Gel cell batteries work well for deep cycling since the particles get suspended in the gel and don’t settle to the bottom of the battery causing the gradual cell shorting.
So back the matter at hand …… Solar panels typically put out low current levels typical to trickle or float chargers…..not suitable for flooded cell batteries (application mismatch).
I recommend you consider investing in gel cell batteries or lithium ion batteries. Lithium ion batteries are typically used in battery operated electric power tools (drills, sawsalls, etc.). Many tool suppliers provide 9.6 volt and 18 volt batteries reasonably priced that will better suit your application of long deep cycle discharges and even longer recharge cycles. You may need to parallel multiple strings of lithium ion batteries to meet your power (volts x amps = watts) needs. In the long run …….. lithium ion batteries and/or gel cell batteries will provide power to your application much better and provide much better performance overall and WILL last much longer than flooded cell batteries in this application!
Hope this helps ……. Good Luck AND Go Green!